Toxic microalgae

Most recently, I’ve been working on an unpleasant example of microbial niche construction on a grand scale: the bloom-forming microalga Prymnesium parvum. Toxicity in this (and other species of algae) is often seen as part of a collective competitive strategy of ‘chemical warfare’–niche construction on a truly massive scale. However, P. parvum is a voracious hunter of other algae, which it attacks and consumes as an alternative nutrient source. Toxicity seems to be linked to this hunting behavior, so that individual cells might benefit from their own toxins (like a rattlesnake), when they are used to capture bacteria or other small prey. However, the entire population benefits when the toxins are used to kill competing algae,

A small swarm of toxic P. parvum (small golden cells) attacks a rotifer accustomed to grazing on unicellular algae. The toxic algae help themselves to a meal, and in the process help all of the algae that the rotifer might have otherwise eaten.

or even large predators like the rotifer pictured below. Thus, the impacts of toxicity can be local or global, depending on ecological context, which changes dramatically over the course of a bloom, which may last several months.

I have isolated several strains that behave very differently, even though they are from the same bloom: some grow slowly and are highly toxic, at least one other grows rapidly, but is only slightly toxic and a poor hunter. Is this strain a ‘cheater’, or simply occupying a separate niche from the toxic hunters? I am using these diverse strains to test ideas about the adaptive value of toxicity in this species and to understand its role in mediating interactions between P. parvum and other plankton.

More generally, I’m having a great time using P. parvum as a highly tractable system for observing basic ecology in the laboratory. Compared with studying ecology in macrobes (as fun as that is), it is relatively easy to observe ecological interactions among microalgae–not just through direct observations of individual- or group-level encounters under a microscope, but at the population-level, through measurements of population growth rates over mere weeks.